Excellent ion selectivity of Nafion membrane modified by PBI via acid-base pair effect for vanadium flow battery

Nafion-Based Proton Exchange Membranes for Vanadium Redox Flow Batteries

The sustainable development of future societies depends on advanced energy storage technologies. Vanadium redox flow batteries (VRFBs) are a preferred solution for large-scale, long-duration energy storage due to their high capacity, long lifespan, rapid response, and safety. The proton exchange membrane (PEM) is a pivotal component of VRFBs, playing a crucial role for conducting protons and preventing vanadium ion crossover. Currently, perfluorinated sulfonic acid membranes, represented by Nafion, are the most commonly used PEMs in VRFBs. However, the size discrepancy between vanadium ions (~0.6 nm) and the ionic domains in Nafion membranes (3-5 nm) leads to significant vanadium permeability, resulting in reduced battery performance. Therefore, rationally regulating the structure of Nafion membranes to enhance their conductive selectivity is an urgent issue. This review focuses on recent advancements in Nafion modification, offering valuable insights for inspiring the fundamental innovation of high-selective Nafion membranes for VRFB technology.

In Situ Crosslinking of Tröger Base-Based Membranes with Improved Vanadium Flow Battery Property

The high conductivity of anion exchange membrane (AEM) remains a great challenge in achieving high-performance vanadium flow batteries. In this work, this is achieved by designing a series of microporous crosslinked quaternary ammonium membranes (QDTTB-Xs), which is synthesized by in situ reacting of iodomethane with a series of novel crosslinked microporous Tröger base membranes (DTTB-Xs) that prepared by condensation of 2, 6 (7)-diamino-triptycene and 2, 6 (7)-13-triamino-triptycene through in situ crosslinking. Compared with linear microporous QDTTB-0, the crosslinked QDTTB-X membranes showed higher conductivity. The QDTTB-35 membrane displays both higher coulombic efficiency and voltage efficiency, and 80% of energy efficiency is realized at 200 mA cm-2. Outperforming N117 and other reported anion exchange membranes. This is due to the increased triamino-triptycene molar ratio in the membrane resulting in both higher N+ concentration and improved micropores concentration. Moreover, positively charged N+ groups combined with the low swelling ratio also help in restricting the vanadium ions permeation. These results give great perspectives in designing high-performance AEMs for VRFB applications.

Ionic-Nanophase Hybridization of Nafion by Supramolecular Patching for Enhanced Proton Selectivity in Redox Flow Batteries

Nafion, as the mostly used proton exchange membrane material in vanadium redox flow batteries (VRFBs), encounters serious vanadium permeation problems due to the large size difference between its anionic nanophase (3-5 nm) and cationic vanadium ions (∼0.6 nm). Bulk hybridization usually suppresses the vanadium permeation at the expense of proton conductivity since conventional additives tend to randomly agglomerate and damage the nanophase continuity from unsuitable sizes and intrinsic incompatibility. Here, we report the ionic-nanophase hybridization strategy of Nafion membranes by using fluorinated block copolymers (FBCs) and polyoxometalates (POMs) as supramolecular patching additives. The cooperative noncovalent interactions among Nafion, interfacial-active FBCs, and POMs can construct a 1 nm-shrunk ionic nanophase with abundant proton transport sites, preserved continuity, and efficient vanadium screeners, which leads to a comprehensive enhancement in proton conductivity, selectivity, and VRFB performance. These results demonstrate the intriguing potential of the supramolecular patching strategy in precisely tuning nanostructured electrolyte membranes for improved performance.

Nanocomposite Membranes Based on Fluoropolymers for Electrochemical Energy Sources

The physicochemical and transport properties (ion-exchange capacity, water content, diffusion permeability, conductivity, and current-voltage characteristic) of a series of perfluorinated membranes with an inert fluoropolymer content from 0 to 40%, obtained by polymer solution casting, were studied. Based on the analysis of the parameters of the extended three-wire model, the effect of an inert component on the path of electric current flow in the membrane and its selectivity were estimated. The mechanical characteristics of the membranes, such as Young's modulus, yield strength, tensile strength, and relative elongation, were determined from the dynamometric curves. The optimal amount of the inert polymer in the perfluorinated membrane was found to be 20%, which does not significantly affect its structure and electrotransport properties but increases the elasticity of the obtained samples. Therefore, the perfluorinated membrane with 20% of inert fluoropolymer is promising for its application in redox flow batteries and direct methanol fuel cells.